1. Technical Field
The present invention relates to a charging member cleaning roller which is applied to an image forming apparatus for forming an image by the electrophotographic method, and cleans a charging member for charging an image holding member. Moreover, the present invention relates to a charging device, a process cartridge, and an image forming apparatus using the charging member cleaning roller.
2. Related Art
A photoreceptor drum is used as the image holding member in image forming apparatuses such as electrophotographic copiers and laser beam printers, and in electrostatic recording apparatuses, which form a toner image on the surface of an image holding member and transfer the toner image onto paper by an electrophotographic method. Moreover, a corotron member is arranged with respect to the photoreceptor drum so that the surface of the photoreceptor drum is charged by the electric discharge from the corotron. However, when the corotron is used, in order to charge the surface of the photoreceptor drum to a predetermined potential, it is necessary to apply a high voltage of several kV, which causes a problem in that ozone is generated by the corona discharge, and the ozone causes deterioration of the photoreceptors or any rubber provided in the image forming apparatus. Therefore, in such an image forming apparatus using the corotron as described above, in addition to selecting the material and shape of members used in the apparatus, it is necessary to provide an exhaust fan for discharging the ozone to the outside of the apparatus, and to provide an auxiliary device which makes the ozone contained in the emission gas harmless.
Moreover, in an apparatus provided with such a corona discharge means, there is a problem in that, if dust in the air, fixing oil, or the like float inside the apparatus, the corotron wire gets dirty and the photoreceptor drum is not uniformly charged, so that image unevenness easily occurs. Here, in some cases, in order to periodically clean the corotron wire so as to prevent nonuniformity of the electric discharge, a device for periodically cleaning the wire is used by arranging a cleaning device or the like for the corotron wire. However, providing a cleaning device or the like for the corotron wire involves additionally providing an extra device, causing a problem in terms of space. Moreover, attaching an extra device to a small photoreceptor drum or the like causes a problem of increasing the constraints for designing the apparatus. In order to solve such an inconvenience in the case of using the corotron wire as described above, devices are drawing attention in which a charging member such as a charging roller is used to charge the photoreceptor drum by bringing into contact therewith.
Furthermore, mechanisms which use a charging member such as a charging roller generally utilize pulse discharge. However, since the image holding member surface is charged by a pulse discharge generated in a minute space between the image holding member and the charging roller, the state of the image holding member surface becomes one of being continuously etched, resulting in further acceleration of film wearing of the image holding member, and having an adverse effect of becoming unsuable with long term usage in some cases.
Here, in order to suppress the pulse discharge so as to reduce the film wearing of the image holding member, a charging device is proposed in which charging involving applying a direct current voltage (DC) (hereunder, DC charging) is utilized as follows.
In this method, the image holding member surface is charged only by a direct current voltage and no alternating voltage is applied. Therefore the amount of current flowing into the image holding member is very small. That is, it means that the pulse discharge into the image holding member is also decreased, and it is considered that, as a result, the etching effect on the image holding member (photoreceptor) is reduced and the film wearing of the image holding member is kept small.
However, the following problems are present in DC charging.
The amount of discharging current in DC charging is determined by the resistance value of the contact charging roller, and the state of the pulse discharge greatly depends on the surface nature of the contact charging roller. Consequently, in order to uniformly charge the image holding member surface only by applying a direct current voltage, a more uniform electrical resistance and a surface smoothness are required compared to the case of superimposing an alternating voltage. Consequently, in order to improve the uniformity of the electrical resistance and the surface smoothness, a cost increment is unavoidable. Moreover, in DC charging, dirt (mainly external additives of the toner) is easily adhered onto the charging roller surface, and the adhered dirt changes the electrical resistance and the surface state, thereby easily interfering with uniform charging. The reason is considered that, since no alternating voltage is applied, no oscillating electric field is generated, and thus the dirt adhered onto the charging roller is hard to remove.
Therefore, in order to keep satisfactory images for a long time while reducing the film wearing of the image holding member by DC charging, a device for cleaning the charging roller or a device for preventing dirt from being adhered onto the charging roller is essential.
With respect to the above problem, there are attempted: a method of forming the surface layer of the charging roller from a highly releasable material; a method of applying a reverse bias that is opposite to the charging polarity at each of fixed timings, so as to reverse the polarity of a residue toner to move toward the image holding member; and so forth.